Different drugs used to inhibit microbial growth act by inhibiting different targets. For example, the fluoroquinolone class of antibiotics act by inhibiting bacterial DNA synthesis. When used in treatment, fluoroquinolones are well absorbed orally, are found in respiratory secretions in higher concentrations than in serum and are concentrated inside macrophages. In addition, fluoroquinolones are well tolerated and have an excellent safety record in long-term therapy.
Antibiotic resistance, and in particular resistance to fluoroquinolones, has become a problem. Fluoroquinolone resistance in gram negative bacteria is principally caused by mutations affecting the target proteins of the drugs. In the case of fluoroquinolones, these targets are DNA gyrase and topoisomerase IV. In addition, mutations affecting regulatory genes such as marA, soxS or rob can cause fluoroquinolone resistance (Oethinger et al. 1998. J. Antimicrob. Chemother. 41:111). Mar A is a transcriptional activator encoded by the marRAB operon involved in multiple antibiotic resistance (Alekshun et al. (1997) Antimicrob. Agents Chemother. 41, 2067–2075). The marRAB locus confers resistance to tetracycline, chloramphenicol, fluoroquinolones, nalidixic acid, rifampin, penicillin, as well as other compounds. However, marRAB does not encode a multidrug efflux system. Rather, it controls the expression of other loci important in directly mediating drug resistance, e.g., ompF, the gene for outer membrane porin, and the acrAB genes for the AcrAB efflux proteins.
AcrAB is a multidrug efflux pump (Nikaido, H. (1996) J. Bacteriol. 178, 5853–5859; Okusu et al. (1996) J. Bacteriol. 178, 306–308) whose normal physiological role is unknown, although it may assist in protection of cells against bile salts in the mammalian small intestine (Thanassi et al. (1997) J. Bacteriol. 179, 2512–2518). The AcrAB operon is upregulated by MarA (Ma et al. (1995) Mol. Microbiol. 16, 45–55). Mutations in the repressor gene marR lead to overexpression of marA (Alekshun et al. (1997). Antimicrob. Agents Chemother. 41, 2067–2075; Cohen et al. (1993) J. Bacteriol. 175, 1484–492); Seoane et al. (1995) J. Bacteriol. 177, 3414–3419). The soxS gene encodes a MarA homolog (Alekshun et al. (1997) Antimicrob. Agents Chemother. 41, 2067–2075; Li et al. (1996) Mol. Microbiol. 20, 937–945; Miller et al. (1996) Mol. Microbiol. 21, 441–448) which also positively regulates acrAB (Ma et al. (1996) Mol. Microbiol. 19, 101–112).
The AcrAB pump primarily controls resistance to large, lipophilic agents that have difficulty penetrating porin channels, such as erythromycin, fusidic acid, dyes, and detergents, while leaving microbes susceptible to small antibiotics that can diffuse through the channel, e.g., tetracycline, chloramphenicol, and fluoroquinolones (Nikaido. 1996. J. Bacteriology 178:5853). Recently, the AcrAB pump has been found to be important in mediating resistance to other drugs used to control microbial growth, e.g., non-antibiotic agents such as triclosan (FEMS Microbiol Lett 1998 Sep. 15; 166: 305–9.
Microbes often become resistant to antibiotics and/or non-antibiotic agents. This can occur by the acquisition of genes encoding enzymes that inactivate the agents, modify the target of the agent, or result in active efflux of the agent. Enzymes that inactivate synthetic antibiotics such as quinolones, sulfonamides, and trimethoprim have not been found. In the case of these antibiotics and natural products for which inactivating or modifying enzymes have not emerged, resistance usually arises by target modifications (Spratt. 1994. Science 264:388). Improved methods for controlling drug resistance in microbes, in particular in microbes that are highly resistant to drugs, would be of tremendous benefit.